# Standard library header <iterator>

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< cpp‎ | header

This header is part of the iterator library.

This header is a partial freestanding header. Everything inside this header is freestanding beside stream iterators. | (since C++23)

### Concepts
##### Iterator concepts
indirectly_readable (C++20) | specifies that a type is indirectly readable by applying operator `*` (concept)
indirectly_writable (C++20) | specifies that a value can be written to an iterator's referenced object (concept)
weakly_incrementable (C++20) | specifies that a `semiregular` type can be incremented with pre- and post-increment operators (concept)
incrementable (C++20) | specifies that the increment operation on a `weakly_incrementable` type is equality-preserving and that the type is `equality_comparable` (concept)
input_or_output_iterator (C++20) | specifies that objects of a type can be incremented and dereferenced (concept)
sentinel_for (C++20) | specifies a type is a sentinel for an `input_or_output_iterator` type (concept)
sized_sentinel_for (C++20) | specifies that the - operator can be applied to an iterator and a sentinel to calculate their difference in constant time (concept)
input_iterator (C++20) | specifies that a type is an input iterator, that is, its referenced values can be read and it can be both pre- and post-incremented (concept)
output_iterator (C++20) | specifies that a type is an output iterator for a given value type, that is, values of that type can be written to it and it can be both pre- and post-incremented (concept)
forward_iterator (C++20) | specifies that an `input_iterator` is a forward iterator, supporting equality comparison and multi-pass (concept)
bidirectional_iterator (C++20) | specifies that a `forward_iterator` is a bidirectional iterator, supporting movement backwards (concept)
random_access_iterator (C++20) | specifies that a `bidirectional_iterator` is a random-access iterator, supporting advancement in constant time and subscripting (concept)
contiguous_iterator (C++20) | specifies that a `random_access_iterator` is a contiguous iterator, referring to elements that are contiguous in memory (concept)
##### Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20) | specifies that a callable type can be invoked with the result of dereferencing an `indirectly_readable` type (concept)
indirect_unary_predicate (C++20) | specifies that a callable type, when invoked with the result of dereferencing an `indirectly_readable` type, satisfies `predicate` (concept)
indirect_binary_predicate (C++20) | specifies that a callable type, when invoked with the result of dereferencing two `indirectly_readable` types, satisfies `predicate` (concept)
indirect_equivalence_relation (C++20) | specifies that a callable type, when invoked with the result of dereferencing two `indirectly_readable` types, satisfies `equivalence_relation` (concept)
indirect_strict_weak_order (C++20) | specifies that a callable type, when invoked with the result of dereferencing two `indirectly_readable` types, satisfies `strict_weak_order` (concept)
##### Common algorithm requirements
indirectly_movable (C++20) | specifies that values may be moved from an `indirectly_readable` type to an `indirectly_writable` type (concept)
indirectly_movable_storable (C++20) | specifies that values may be moved from an `indirectly_readable` type to an `indirectly_writable` type and that the move may be performed via an intermediate object (concept)
indirectly_copyable (C++20) | specifies that values may be copied from an `indirectly_readable` type to an `indirectly_writable` type (concept)
indirectly_copyable_storable (C++20) | specifies that values may be copied from an `indirectly_readable` type to an `indirectly_writable` type and that the copy may be performed via an intermediate object (concept)
indirectly_swappable (C++20) | specifies that the values referenced by two `indirectly_readable` types can be swapped (concept)
indirectly_comparable (C++20) | specifies that the values referenced by two `indirectly_readable` types can be compared (concept)
permutable (C++20) | specifies the common requirements of algorithms that reorder elements in place (concept)
mergeable (C++20) | specifies the requirements of algorithms that merge sorted sequences into an output sequence by copying elements (concept)
sortable (C++20) | specifies the common requirements of algorithms that permute sequences into ordered sequences (concept)
### Classes
##### Algorithm utilities
indirect_result_t (C++20) | computes the result of invoking a callable object on the result of dereferencing some set of `indirectly_readable` types (alias template)
projected (C++20) | helper template for specifying the constraints on algorithms that accept projections (class template)
projected_value_t (C++26) | computes the value type of an `indirectly_readable` type by projection (alias template)
##### Associated types
incrementable_traits (C++20) | computes the difference type of a `weakly_incrementable` type (class template)
indirectly_readable_traits (C++20) | computes the value type of an `indirectly_readable` type (class template)
iter_value_titer_reference_titer_const_reference_titer_difference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++23)(C++20)(C++20)(C++20) | computes the associated types of an iterator (alias template)
##### Primitives
iterator_traits | provides uniform interface to the properties of an iterator (class template)
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20) | empty class types used to indicate iterator categories (class)
iterator (deprecated in C++17) | base class to ease the definition of required types for simple iterators (class template)
##### Adaptors
reverse_iterator | iterator adaptor for reverse-order traversal (class template)
move_iterator (C++11) | iterator adaptor which dereferences to an rvalue (class template)
move_sentinel (C++20) | sentinel adaptor for std::move_iterator (class template)
basic_const_iterator (C++23) | iterator adaptor that converts an iterator into a constant iterator (class template)
const_iterator (C++23) | computes a constant iterator type for a given type (alias template)
const_sentinel (C++23) | computes a sentinel type to be used with constant iterators (alias template)
common_iterator (C++20) | adapts an iterator type and its sentinel into a common iterator type (class template)
default_sentinel_t (C++20) | default sentinel for use with iterators that know the bound of their range (class)
counted_iterator (C++20) | iterator adaptor that tracks the distance to the end of the range (class template)
unreachable_sentinel_t (C++20) | sentinel that always compares unequal to any `weakly_incrementable` type (class)
back_insert_iterator | iterator adaptor for insertion at the end of a container (class template)
front_insert_iterator | iterator adaptor for insertion at the front of a container (class template)
insert_iterator | iterator adaptor for insertion into a container (class template)
##### Stream Iterators
istream_iterator | input iterator that reads from std::basic_istream (class template)
ostream_iterator | output iterator that writes to std::basic_ostream (class template)
istreambuf_iterator | input iterator that reads from std::basic_streambuf (class template)
ostreambuf_iterator | output iterator that writes to std::basic_streambuf (class template)
### Customization point objects
Defined in namespace `std::ranges`
iter_move (C++20) | casts the result of dereferencing an object to its associated rvalue reference type (customization point object)
iter_swap (C++20) | swaps the values referenced by two dereferenceable objects (customization point object)
### Constants
unreachable_sentinel (C++20) | an object of type `unreachable_sentinel_t` that always compares unequal to any `weakly_incrementable` type (constant)
default_sentinel (C++20) | an object of type `default_sentinel_t` used with iterators that know the bound of their range (constant)
### Functions
##### Adaptors
make_reverse_iterator (C++14) | creates a std::reverse_iterator of type inferred from the argument (function template)
make_move_iterator (C++11) | creates a std::move_iterator of type inferred from the argument (function template)
make_const_iterator (C++23) | creates a std::const_iterator of type inferred from the argument (function template)
make_const_sentinel (C++23) | creates a std::const_sentinel of type inferred from the argument (function template)
front_inserter | creates a std::front_insert_iterator of type inferred from the argument (function template)
back_inserter | creates a std::back_insert_iterator of type inferred from the argument (function template)
inserter | creates a std::insert_iterator of type inferred from the argument (function template)
##### Non-member operators
operator==operator!=operator<operator<=operator>operator>=operator<=> (C++11)(C++11)(removed in C++20)(C++11)(C++11)(C++11)(C++11)(C++20) | compares the underlying iterators (function template)
operator+ (C++11) | advances the iterator (function template)
operator- (C++11) | computes the distance between two iterator adaptors (function template)
operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20) | compares the underlying iterators (function template)
operator+ | advances the iterator (function template)
operator- | computes the distance between two iterator adaptors (function template)
operator==operator<=> (C++20) | compares the distances to the end (function template)
operator+ (C++20) | advances the iterator (function template)
operator- (C++20) | computes the distance between two iterator adaptors (function template)
operator==operator!= (removed in C++20) | compares two `istream_iterator`s (function template)
operator==operator!= (removed in C++20) | compares two `istreambuf_iterator`s (function template)
##### Operations
advance | advances an iterator by given distance (function template)
distance | returns the distance between two iterators (function template)
next (C++11) | increment an iterator (function template)
prev (C++11) | decrement an iterator (function template)
ranges::advance (C++20) | advances an iterator by given distance or to a given bound (algorithm function object)
ranges::distance (C++20) | returns the distance between an iterator and a sentinel, or between the beginning and end of a range (algorithm function object)
ranges::next (C++20) | increment an iterator by a given distance or to a bound (algorithm function object)
ranges::prev (C++20) | decrement an iterator by a given distance or to a bound (algorithm function object)
##### Range access
begincbegin (C++11)(C++14) | returns an iterator to the beginning of a container or array (function template)
endcend (C++11)(C++14) | returns an iterator to the end of a container or array (function template)
rbegincrbegin (C++14) | returns a reverse iterator to the beginning of a container or array (function template)
rendcrend (C++14) | returns a reverse end iterator for a container or array (function template)
sizessize (C++17)(C++20) | returns the size of a container or array (function template)
empty (C++17) | checks whether the container is empty (function template)
data (C++17) | obtains the pointer to the underlying array (function template)

### Synopsis

    #include <compare>
    #include <concepts>

    namespace std {
      template<class T> using /* with-reference */ = T&;  // exposition only
      template<class T> concept /* can-reference */       // exposition only
        = requires { typename /* with-reference */<T>; };
      template<class T> concept /* dereferenceable */     // exposition only
        = requires(T& t) {
          { *t } -> /* can-reference */;  // not required to be equality-preserving
        };

      // associated types
      // incrementable traits
      template<class> struct incrementable_traits;
      template<class T>
        using iter_difference_t = /* see description */;

      // indirectly readable traits
      template<class> struct indirectly_readable_traits;
      template<class T>
        using iter_value_t = /* see description */;

      // iterator traits
      template<class I> struct iterator_traits;
      template<class T> requires is_object_v<T> struct iterator_traits<T*>;

      template</* dereferenceable */ T>
        using iter_reference_t = decltype(*declval<T&>());

      namespace ranges {
        // customization point objects
        inline namespace /* unspecified */ {
          // ranges::iter_move
          inline constexpr /* unspecified */ iter_move = /* unspecified */;

          // ranges::iter_swap
          inline constexpr /* unspecified */ iter_swap = /* unspecified */;
        }
      }

      template</* dereferenceable */ T>
        requires requires(T& t) {
          { ranges::iter_move(t) } -> /* can-reference */;
        }
      using iter_rvalue_reference_t
        = decltype(ranges::iter_move(declval<T&>()));

      // iterator concepts
      // concept indirectly_readable
      template<class In>
        concept indirectly_readable = /* see description */;

      template<indirectly_readable T>
        using iter_common_reference_t =
          common_reference_t<iter_reference_t<T>, iter_value_t<T>&>;

      // concept indirectly_writable
      template<class Out, class T>
        concept indirectly_writable = /* see description */;

      // concept weakly_incrementable
      template<class I>
        concept weakly_incrementable = /* see description */;

      // concept incrementable
      template<class I>
        concept incrementable = /* see description */;

      // concept input_or_output_iterator
      template<class I>
        concept input_or_output_iterator = /* see description */;

      // concept sentinel_for
      template<class S, class I>
        concept sentinel_for = /* see description */;

      // concept sized_sentinel_for
      template<class S, class I>
        inline constexpr bool disable_sized_sentinel_for = false;

      template<class S, class I>
        concept sized_sentinel_for = /* see description */;

      // concept input_iterator
      template<class I>
        concept input_iterator = /* see description */;

      // concept output_iterator
      template<class I, class T>
        concept output_iterator = /* see description */;

      // concept forward_iterator
      template<class I>
        concept forward_iterator = /* see description */;

      // concept bidirectional_iterator
      template<class I>
        concept bidirectional_iterator = /* see description */;

      // concept random_access_iterator
      template<class I>
        concept random_access_iterator = /* see description */;

      // concept contiguous_iterator
      template<class I>
        concept contiguous_iterator = /* see description */;

      // indirect callable requirements
      // indirect callables
      template<class F, class I>
        concept indirectly_unary_invocable = /* see description */;

      template<class F, class I>
        concept indirectly_regular_unary_invocable = /* see description */;

      template<class F, class I>
        concept indirect_unary_predicate = /* see description */;

      template<class F, class I1, class I2>
        concept indirect_binary_predicate = /* see description */;

      template<class F, class I1, class I2 = I1>
        concept indirect_equivalence_relation = /* see description */;

      template<class F, class I1, class I2 = I1>
        concept indirect_strict_weak_order = /* see description */;

      template<class F, class... Is>
        requires (indirectly_readable<Is> && ...) && invocable<F, iter_reference_t<Is>...>
          using indirect_result_t = invoke_result_t<F, iter_reference_t<Is>...>;

      // projected
      template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj>
        struct projected;

      template<weakly_incrementable I, class Proj>
        struct incrementable_traits<projected<I, Proj>>;

      template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj>
        using projected_value_t = remove_cvref_t<invoke_result_t<Proj&, iter_value_t<I>&>>;

      // common algorithm requirements
      // concept indirectly_movable
      template<class In, class Out>
        concept indirectly_movable = /* see description */;

      template<class In, class Out>
        concept indirectly_movable_storable = /* see description */;

      // concept indirectly_copyable
      template<class In, class Out>
        concept indirectly_copyable = /* see description */;

      template<class In, class Out>
        concept indirectly_copyable_storable = /* see description */;

      // concept indirectly_swappable
      template<class I1, class I2 = I1>
        concept indirectly_swappable = /* see description */;

      // concept indirectly_comparable
      template<class I1, class I2, class R, class P1 = identity, class P2 = identity>
        concept indirectly_comparable = /* see description */;

      // concept permutable
      template<class I>
        concept permutable = /* see description */;

      // concept mergeable
      template<class I1, class I2, class Out,
          class R = ranges::less, class P1 = identity, class P2 = identity>
        concept mergeable = /* see description */;

      // concept sortable
      template<class I, class R = ranges::less, class P = identity>
        concept sortable = /* see description */;

      // primitives
      // iterator tags
      struct input_iterator_tag { };
      struct output_iterator_tag { };
      struct forward_iterator_tag: public input_iterator_tag { };
      struct bidirectional_iterator_tag: public forward_iterator_tag { };
      struct random_access_iterator_tag: public bidirectional_iterator_tag { };
      struct contiguous_iterator_tag: public random_access_iterator_tag { };

      // iterator operations
      template<class InputIt, class Distance>
        constexpr void advance(InputIt& i, Distance n);
      template<class InputIt>
        constexpr typename iterator_traits<InputIt>::difference_type
          distance(InputIt first, InputIt last);
      template<class InputIt>
        constexpr InputIt
          next(InputIt x, typename iterator_traits<InputIt>::difference_type n = 1);
      template<class BidirIt>
        constexpr BidirIt
          prev(BidirIt x, typename iterator_traits<BidirIt>::difference_type n = 1);

      // range iterator operations
      namespace ranges {
        // ranges::advance
        template<input_or_output_iterator I>
          constexpr void advance(I& i, iter_difference_t<I> n);
        template<input_or_output_iterator I, sentinel_for<I> S>
          constexpr void advance(I& i, S bound);
        template<input_or_output_iterator I, sentinel_for<I> S>
          constexpr iter_difference_t<I> advance(I& i, iter_difference_t<I> n, S bound);

        // ranges::distance
        template<class I, sentinel_for<I> S>
          requires (!sized_sentinel_for<S, I>)
          constexpr iter_difference_t<I> distance(I first, S last);
        template<class I, sized_sentinel_for<decay_t<I>> S>
          constexpr iter_difference_t<decay_t<I>> distance(I&& first, S last);
        template<range R>
          constexpr range_difference_t<R> distance(R&& r);

        // ranges::next
        template<input_or_output_iterator I>
          constexpr I next(I x);
        template<input_or_output_iterator I>
          constexpr I next(I x, iter_difference_t<I> n);
        template<input_or_output_iterator I, sentinel_for<I> S>
          constexpr I next(I x, S bound);
        template<input_or_output_iterator I, sentinel_for<I> S>
          constexpr I next(I x, iter_difference_t<I> n, S bound);

        // ranges::prev
        template<bidirectional_iterator I>
          constexpr I prev(I x);
        template<bidirectional_iterator I>
          constexpr I prev(I x, iter_difference_t<I> n);
        template<bidirectional_iterator I>
          constexpr I prev(I x, iter_difference_t<I> n, I bound);
      }

      // predefined iterators and sentinels
      // reverse iterators
      template<class It> class reverse_iterator;

      template<class It1, class It2>
        constexpr bool operator==(const reverse_iterator<It1>& x,
                                  const reverse_iterator<It2>& y);
      template<class It1, class It2>
        constexpr bool operator!=(const reverse_iterator<It1>& x,
                                  const reverse_iterator<It2>& y);
      template<class It1, class It2>
        constexpr bool operator<(const reverse_iterator<It1>& x,
                                 const reverse_iterator<It2>& y);
      template<class It1, class It2>
        constexpr bool operator>(const reverse_iterator<It1>& x,
                                 const reverse_iterator<It2>& y);
      template<class It1, class It2>
        constexpr bool operator<=(const reverse_iterator<It1>& x,
                                  const reverse_iterator<It2>& y);
      template<class It1, class It2>
        constexpr bool operator>=(const reverse_iterator<It1>& x,
                                  const reverse_iterator<It2>& y);
      template<class It1, three_way_comparable_with<It1> It2>
        constexpr compare_three_way_result_t<It1, It2>
          operator<=>(const reverse_iterator<It1>& x, const reverse_iterator<It2>& y);

      template<class It1, class It2>
        constexpr auto operator-(const reverse_iterator<It1>& x,
                                 const reverse_iterator<It2>& y)
          -> decltype(y.base() - x.base());
      template<class It>
        constexpr reverse_iterator<It> operator+(iter_difference_t<It> n,
                                                 const reverse_iterator<It>& x);

      template<class It>
        constexpr reverse_iterator<It> make_reverse_iterator(It i);

      template<class It1, class It2>
          requires (!sized_sentinel_for<It1, It2>)
        inline constexpr bool disable_sized_sentinel_for<reverse_iterator<It1>,
                                                         reverse_iterator<It2>> = true;

      // insert iterators
      template<class Container> class back_insert_iterator;
      template<class Container>
        constexpr back_insert_iterator<Container> back_inserter(Container& x);

      template<class Container> class front_insert_iterator;
      template<class Container>
        constexpr front_insert_iterator<Container> front_inserter(Container& x);

      template<class Container> class insert_iterator;
      template<class Container>
        constexpr insert_iterator<Container>
          inserter(Container& x, ranges::iterator_t<Container> i);

      // move iterators and sentinels
      template<class It> class move_iterator;

      template<class It1, class It2>
        constexpr bool operator==(const move_iterator<It1>& x, const move_iterator<It2>& y);
      template<class It1, class It2>
        constexpr bool operator<(const move_iterator<It1>& x, const move_iterator<It2>& y);
      template<class It1, class It2>
        constexpr bool operator>(const move_iterator<It1>& x, const move_iterator<It2>& y);
      template<class It1, class It2>
        constexpr bool operator<=(const move_iterator<It1>& x, const move_iterator<It2>& y);
      template<class It1, class It2>
        constexpr bool operator>=(const move_iterator<It1>& x, const move_iterator<It2>& y);
      template<class It1, three_way_comparable_with<It1> It2>
        constexpr compare_three_way_result_t<It1, It2>
          operator<=>(const move_iterator<It1>& x, const move_iterator<It2>& y);

      template<class It1, class It2>
        constexpr auto operator-(const move_iterator<It1>& x, const move_iterator<It2>& y)
          -> decltype(x.base() - y.base());
      template<class It>
        constexpr move_iterator<It>
          operator+(iter_difference_t<It> n, const move_iterator<It>& x);

      template<class It>
        constexpr move_iterator<It> make_move_iterator(It i);

      template<semiregular S> class move_sentinel;

      // common iterators
      template<input_or_output_iterator I, sentinel_for<I> S>
        requires (!same_as<I, S> && copyable<I>)
          class common_iterator;

      template<class I, class S>
        struct incrementable_traits<common_iterator<I, S>>;

      template<input_iterator I, class S>
        struct iterator_traits<common_iterator<I, S>>;

      // default sentinel
      struct default_sentinel_t;
      inline constexpr default_sentinel_t default_sentinel{};

      // counted iterators
      template<input_or_output_iterator I> class counted_iterator;

      template<input_iterator I>
        requires /* see description */
        struct iterator_traits<counted_iterator<I>>;

      // unreachable sentinel
      struct unreachable_sentinel_t;
      inline constexpr unreachable_sentinel_t unreachable_sentinel{};

      // stream iterators
      template<class T, class CharT = char, class Traits = char_traits<CharT>,
               class Distance = ptrdiff_t>
      class istream_iterator;
      template<class T, class CharT, class Traits, class Distance>
        bool operator==(const istream_iterator<T, CharT, Traits, Distance>& x,
                        const istream_iterator<T, CharT, Traits, Distance>& y);

      template<class T, class CharT = char, class traits = char_traits<CharT>>
          class ostream_iterator;

      template<class CharT, class Traits = char_traits<CharT>>
        class istreambuf_iterator;
      template<class CharT, class Traits>
        bool operator==(const istreambuf_iterator<CharT, Traits>& a,
                        const istreambuf_iterator<CharT, Traits>& b);

      template<class CharT, class Traits = char_traits<CharT>>
        class ostreambuf_iterator;

      // range access
      template<class C> constexpr auto begin(C& c) -> decltype(c.begin());
      template<class C> constexpr auto begin(const C& c) -> decltype(c.begin());
      template<class C> constexpr auto end(C& c) -> decltype(c.end());
      template<class C> constexpr auto end(const C& c) -> decltype(c.end());
      template<class T, size_t N> constexpr T* begin(T (&a)[N]) noexcept;
      template<class T, size_t N> constexpr T* end(T (&a)[N]) noexcept;
      template<class C> constexpr auto cbegin(const C& c) noexcept(noexcept(std::begin(c)))
        -> decltype(std::begin(c));
      template<class C> constexpr auto cend(const C& c) noexcept(noexcept(std::end(c)))
        -> decltype(std::end(c));
      template<class C> constexpr auto rbegin(C& c) -> decltype(c.rbegin());
      template<class C> constexpr auto rbegin(const C& c) -> decltype(c.rbegin());
      template<class C> constexpr auto rend(C& c) -> decltype(c.rend());
      template<class C> constexpr auto rend(const C& c) -> decltype(c.rend());
      template<class T, size_t N> constexpr reverse_iterator<T*> rbegin(T (&a)[N]);
      template<class T, size_t N> constexpr reverse_iterator<T*> rend(T (&a)[N]);
      template<class E> constexpr reverse_iterator<const E*> rbegin(initializer_list<E> il);
      template<class E> constexpr reverse_iterator<const E*> rend(initializer_list<E> il);
      template<class C> constexpr auto crbegin(const C& c) -> decltype(std::rbegin(c));
      template<class C> constexpr auto crend(const C& c) -> decltype(std::rend(c));

      template<class C> constexpr auto size(const C& c) -> decltype(c.size());
      template<class T, size_t N> constexpr size_t size(const T (&a)[N]) noexcept;
      template<class C> constexpr auto ssize(const C& c)
        -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;
      template<class T, ptrdiff_t N> constexpr ptrdiff_t ssize(const T (&a)[N]) noexcept;
      template<class C> constexpr auto empty(const C& c) -> decltype(c.empty());
      template<class T, size_t N> constexpr bool empty(const T (&a)[N]) noexcept;
      template<class E> constexpr bool empty(initializer_list<E> il) noexcept;
      template<class C> constexpr auto data(C& c) -> decltype(c.data());
      template<class C> constexpr auto data(const C& c) -> decltype(c.data());
      template<class T, size_t N> constexpr T* data(T (&a)[N]) noexcept;
      template<class E> constexpr const E* data(initializer_list<E> il) noexcept;
    }

#### Concept `indirectly_readable`

    namespace std {
      template<class In>
        concept __indirectlyReadableImpl = // exposition only
          requires(const In in) {
            typename iter_value_t<In>;
            typename iter_reference_t<In>;
            typename iter_rvalue_reference_t<In>;
            { *in } -> same_as<iter_reference_t<In>>
            { iter_move(in) } -> same_as<iter_rvalue_reference_t<In>>
          } &&
          common_reference_with<iter_reference_t<In>&&, iter_value_t<In>&> &&
          common_reference_with<iter_reference_t<In>&&, iter_rvalue_reference_t<In>&&> &&
          common_reference_with<iter_rvalue_reference_t<In>&&, const iter_value_t<In>&>;

      template<class In>
        concept indirectly_readable =
          __indirectlyReadableImpl<remove_cvref_t<In>>
    }

#### Concept `indirectly_writable`

    namespace std {
      template<class Out, class T>
        concept indirectly_writable =
          requires(Out&& o, T&& t) {
            *o = std::forward<T>(t); // not required to be equality-preserving
            *std::forward<Out>(o) = std::forward<T>(t);
            // not required to be equality-preserving
            const_cast<const iter_reference_t<Out>&&>(*o) =
            std::forward<T>(t); // not required to be equality-preserving
            const_cast<const iter_reference_t<Out>&&>(*std::forward<Out>(o)) =
            std::forward<T>(t); // not required to be equality-preserving
          };
    }

#### Concept `weakly_incrementable`

    namespace std {
      template<class T>
        inline constexpr bool __is_integer_like = /* see description */; // exposition only

      template<class T>
        inline constexpr bool __is_signed_integer_like =  // exposition only
          /* see description */;

      template<class I>
        concept weakly_incrementable =
          default_initializable<I> && movable<I> &&
          requires(I i) {
            typename iter_difference_t<I>;
            requires __is_signed_integer_like<iter_difference_t<I>>;
            { ++i } -> same_as<I&>;   // not required to be equality-preserving
            i++;                      // not required to be equality-preserving
          };
    }

#### Concept `incrementable`

    namespace std {
      template<class I>
        concept incrementable =
          regular<I> &&
          weakly_incrementable<I> &&
          requires(I i) {
            { i++ } -> same_as<I>;
          };
    }

#### Concept `input_or_output_iterator`

    namespace std {
      template<class I>
        concept input_or_output_iterator =
          requires(I i) {
            { *i } -> can-reference;
          } &&
          weakly_incrementable<I>;
    }

#### Concept `sentinel_for`

    namespace std {
      template<class S, class I>
        concept sentinel_for =
          semiregular<S> &&
          input_or_output_iterator<I> &&
          __WeaklyEqualityComparableWith<S, I>;
    }

#### Concept `sized_sentinel_for`

    namespace std {
      template<class S, class I>
        concept sized_sentinel_for =
          sentinel_for<S, I> &&
          !disable_sized_sentinel<remove_cv_t<S>, remove_cv_t<I>> &&
          requires(const I& i, const S& s) {
            { s - i } -> same_as<iter_difference_t<I>>;
            { i - s } -> same_as<iter_difference_t<I>>;
          };
    }

#### Concept `input_iterator`

    namespace std {
      template<class I>
        concept input_iterator =
          input_or_output_iterator<I> &&
          indirectly_readable<I> &&
          requires { typename /* ITER_CONCEPT */(I); } &&
          derived_from</* ITER_CONCEPT */(I), input_iterator_tag>;
    }

#### Concept `output_iterator`

    namespace std {
      template<class I, class T>
        concept output_iterator =
          input_or_output_iterator<I> &&
          indirectly_writable<I, T> &&
          requires(I i, T&& t) {
            *i++ = std::forward<T>(t); // not required to be equality-preserving
          };
    }

#### Concept `forward_iterator`

    namespace std {
      template<class I>
        concept forward_iterator =
          input_iterator<I> &&
          derived_from</* ITER_CONCEPT */(I), forward_iterator_tag> &&
          incrementable<I> &&
          sentinel_for<I, I>;
    }

#### Concept `bidirectional_iterator`

    namespace std {
      template<class I>
        concept bidirectional_iterator =
          forward_iterator<I> &&
          derived_from</* ITER_CONCEPT */(I), bidirectional_iterator_tag> &&
          requires(I i) {
            { --i } -> same_as<I&>;
            { i-- } -> same_as<I>;
          };
    }

#### Concept `random_access_iterator`

    namespace std {
      template<class I>
        concept random_access_iterator =
          bidirectional_iterator<I> &&
          derived_from</* ITER_CONCEPT */(I), random_access_iterator_tag> &&
          totally_ordered<I> &&
          sized_sentinel_for<I, I> &&
          requires(I i, const I j, const iter_difference_t<I> n) {
            { i += n } -> same_as<I&>;
            { j +  n } -> same_as<I>;
            { n +  j } -> same_as<I>;
            { i -= n } -> same_as<I&>;
            { j -  n } -> same_as<I>;
            {  j[n]  } -> same_as<iter_reference_t<I>>;
          };
    }

#### Concept `contiguous_iterator`

    namespace std {
      template<class I>
        concept contiguous_iterator =
          random_access_iterator<I> &&
          derived_from</* ITER_CONCEPT */(I), contiguous_iterator_tag> &&
          is_lvalue_reference_v<iter_reference_t<I>> &&
          same_as<iter_value_t<I>, remove_cvref_t<iter_reference_t<I>>> &&
          requires(const I& i) {
            { to_address(i) } -> same_as<add_pointer_t<iter_reference_t<I>>>;
          };
    }

#### Concept `indirectly_unary_invocable`

    namespace std {
      template<class F, class I>
        concept indirectly_unary_invocable =
          indirectly_readable<I> &&
          copy_constructible<F> &&
          invocable<F&, iter_value_t<I>&> &&
          invocable<F&, iter_reference_t<I>> &&
          common_reference_with<
            invoke_result_t<F&, iter_value_t<I>&>,
            invoke_result_t<F&, iter_reference_t<I>>>;
    }

#### Concept `indirectly_regular_unary_invocable`

    namespace std {
      template<class F, class I>
        concept indirectly_regular_unary_invocable =
          indirectly_readable<I> &&
          copy_constructible<F> &&
          regular_invocable<F&, iter_value_t<I>&> &&
          regular_invocable<F&, iter_reference_t<I>> &&
          common_reference_with<
            invoke_result_t<F&, iter_value_t<I>&>,
            invoke_result_t<F&, iter_reference_t<I>>>;
    }

#### Concept `indirect_unary_predicate`

    namespace std {
      template<class F, class I>
        concept indirect_unary_predicate =
          indirectly_readable<I> &&
          copy_constructible<F> &&
          predicate<F&, iter_value_t<I>&> &&
          predicate<F&, iter_reference_t<I>>;
    }

#### Concept `indirect_binary_predicate`

    namespace std {
      template<class F, class I1, class I2 = I1>
        concept indirect_binary_predicate =
          indirectly_readable<I1> && indirectly_readable<I2> &&
          copy_constructible<F> &&
          predicate<F&, iter_value_t<I1>&, iter_value_t<I2>&> &&
          predicate<F&, iter_value_t<I1>&, iter_reference_t<I2>> &&
          predicate<F&, iter_reference_t<I1>, iter_value_t<I2>&> &&
          predicate<F&, iter_reference_t<I1>, iter_reference_t<I2>>;
    }

#### Concept `indirect_equivalence_relation`

    namespace std {
      template<class F, class I1, class I2 = I1>
        concept indirect_equivalence_relation =
          indirectly_readable<I1> && indirectly_readable<I2> &&
          copy_constructible<F> &&
          equivalence_relation<F&, iter_value_t<I1>&, iter_value_t<I2>&> &&
          equivalence_relation<F&, iter_value_t<I1>&, iter_reference_t<I2>> &&
          equivalence_relation<F&, iter_reference_t<I1>, iter_value_t<I2>&> &&
          equivalence_relation<F&, iter_reference_t<I1>, iter_reference_t<I2>>;
    }

#### Concept `indirect_strict_weak_order`

    namespace std {
      template<class F, class I1, class I2 = I1>
        concept indirect_strict_weak_order =
          indirectly_readable<I1> && indirectly_readable<I2> &&
          copy_constructible<F> &&
          strict_weak_order<F&, iter_value_t<I1>&, iter_value_t<I2>&> &&
          strict_weak_order<F&, iter_value_t<I1>&, iter_reference_t<I2>> &&
          strict_weak_order<F&, iter_reference_t<I1>, iter_value_t<I2>&> &&
          strict_weak_order<F&, iter_reference_t<I1>, iter_reference_t<I2>>;
    }

#### Concept `indirectly_movable`

    namespace std {
      template<class In, class Out>
        concept indirectly_movable =
          indirectly_readable<In> &&
          indirectly_writable<Out, iter_rvalue_reference_t<In>>;
    }

#### Concept `indirectly_movable_storable`

    namespace std {
      template<class In, class Out>
        concept indirectly_movable_storable =
          indirectly_movable<In, Out> &&
          indirectly_writable<Out, iter_value_t<In>> &&
          movable<iter_value_t<In>> &&
          constructible_from<iter_value_t<In>, iter_rvalue_reference_t<In>> &&
          assignable_from<iter_value_t<In>&, iter_rvalue_reference_t<In>>;
    }

#### Concept `indirectly_copyable`

    namespace std {
      template<class In, class Out>
        concept indirectly_copyable =
          indirectly_readable<In> &&
          indirectly_writable<Out, iter_reference_t<In>>;
    }

#### Concept `indirectly_copyable_storable`

    namespace std {
      template<class In, class Out>
        concept indirectly_copyable_storable =
          indirectly_copyable<In, Out> &&
          indirectly_writable<Out, iter_value_t<In>&> &&
          indirectly_writable<Out, const iter_value_t<In>&> &&
          indirectly_writable<Out, iter_value_t<In>&&> &&
          indirectly_writable<Out, const iter_value_t<In>&&> &&
          copyable<iter_value_t<In>> &&
          constructible_from<iter_value_t<In>, iter_reference_t<In>> &&
          assignable_from<iter_value_t<In>&, iter_reference_t<In>>;
    }

#### Concept `indirectly_swappable`

    namespace std {
      template<class I1, class I2 = I1>
        concept indirectly_swappable =
          indirectly_readable<I1> && indirectly_readable<I2> &&
          requires(const I1 i1, const I2 i2) {
            ranges::iter_swap(i1, i1);
            ranges::iter_swap(i2, i2);
            ranges::iter_swap(i1, i2);
            ranges::iter_swap(i2, i1);
          };
    }

#### Concept `indirectly_comparable`

    namespace std {
      template<class I1, class I2, class R, class P1 = identity, class P2 = identity>
        concept indirectly_comparable =
          indirect_predicate<R, projected<I1, P1>, projected<I2, P2>>;
    }

#### Concept `permutable`

    namespace std {
      template<class I>
        concept permutable =
          forward_iterator<I> &&
          indirectly_movable_storable<I, I> &&
          indirectly_swappable<I, I>;
    }

#### Concept `mergeable`

    namespace std {
      template<class I1, class I2, class Out, class R = ranges::less,
               class P1 = identity, class P2 = identity>
        concept mergeable =
          input_iterator<I1> &&
          input_iterator<I2> &&
          weakly_incrementable<Out> &&
          indirectly_copyable<I1, Out> &&
          indirectly_copyable<I2, Out> &&
          indirect_strict_weak_order<R, projected<I1, P1>, projected<I2, P2>>;
    }

#### Concept `sortable`

    namespace std {
      template<class I, class R = ranges::less, class P = identity>
        concept sortable =
          permutable<I> &&
          indirect_strict_weak_order<R, projected<I, P>>;
    }

#### Class template std::incrementable_traits

    namespace std {
      template<class> struct incrementable_traits { };

      template<class T>
        requires is_object_v<T>
      struct incrementable_traits<T*> {
        using difference_type = ptrdiff_t;
      };

      template<class I>
      struct incrementable_traits<const I>
        : incrementable_traits<I> { };

      template<class T>
        requires requires { typename T::difference_type; }
      struct incrementable_traits<T> {
        using difference_type = typename T::difference_type;
      };

      template<class T>
        requires (!requires { typename T::difference_type; } &&
                  requires(const T& a, const T& b) { { a - b } -> integral; })
      struct incrementable_traits<T> {
        using difference_type = make_signed_t<decltype(declval<T>() - declval<T>())>;
      };

      template<class T>
        using iter_difference_t = /* see description */;
    }

#### Class template std::indirectly_readable_traits

    namespace std {
      template<class> struct __cond_value_type { };   // exposition only
      template<class T>
        requires is_object_v<T>
      struct __cond_value_type {
        using value_type = remove_cv_t<T>;
      };

      template<class> struct indirectly_readable_traits { };

      template<class T>
      struct indirectly_readable_traits<T*>
        : __cond_value_type<T> { };

      template<class I>
        requires is_array_v<I>
      struct indirectly_readable_traits<I> {
        using value_type = remove_cv_t<remove_extent_t<I>>;
      };

      template<class I>
      struct indirectly_readable_traits<const I>
        : indirectly_readable_traits<I> { };

      template<class T>
        requires requires { typename T::value_type; }
      struct indirectly_readable_traits<T>
        : __cond_value_type<typename T::value_type> { };

      template<class T>
        requires requires { typename T::element_type; }
      struct indirectly_readable_traits<T>
        : __cond_value_type<typename T::element_type> { };
    }

#### Class template std::projected

    namespace std {
      template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj>
      struct projected {
        using value_type = remove_cvref_t<indirect_result_t<Proj&, I>>;
        indirect_result_t<Proj&, I> operator*() const; // not defined
      };

      template<weakly_incrementable I, class Proj>
      struct incrementable_traits<projected<I, Proj>> {
        using difference_type = iter_difference_t<I>;
      };
    }

#### Class template std::iterator_traits

    namespace std {
      template<class I>
      struct iterator_traits {
        using iterator_category = /* see description */;
        using value_type        = /* see description */;
        using difference_type   = /* see description */;
        using pointer           = /* see description */;
        using reference         = /* see description */;
      };

      template<class T>
        requires is_object_v<T>
      struct iterator_traits<T*> {
        using iterator_concept  = contiguous_iterator_tag;
        using iterator_category = random_access_iterator_tag;
        using value_type        = remove_cv_t<T>;
        using difference_type   = ptrdiff_t;
        using pointer           = T*;
        using reference         = T&;
      };
    }

#### Iterator tags

    namespace std {
      struct input_iterator_tag { };
      struct output_iterator_tag { };
      struct forward_iterator_tag: public input_iterator_tag { };
      struct bidirectional_iterator_tag: public forward_iterator_tag { };
      struct random_access_iterator_tag: public bidirectional_iterator_tag { };
      struct contiguous_iterator_tag: public random_access_iterator_tag { };
    }

#### Class template std::reverse_iterator

    namespace std {
      template<class Iter>
      class reverse_iterator {
      public:
        using iterator_type     = Iter;
        using iterator_concept  = /* see description */;
        using iterator_category = /* see description */;
        using value_type        = iter_value_t<Iter>;
        using difference_type   = iter_difference_t<Iter>;
        using pointer           = typename iterator_traits<Iter>::pointer;
        using reference         = iter_reference_t<Iter>;

        constexpr reverse_iterator();
        constexpr explicit reverse_iterator(Iter x);
        template<class U> constexpr reverse_iterator(const reverse_iterator<U>& u);
        template<class U> constexpr reverse_iterator& operator=(const reverse_iterator<U>& u);

        constexpr Iter base() const;
        constexpr reference operator*() const;
        constexpr pointer   operator->() const requires /* see description */;

        constexpr reverse_iterator& operator++();
        constexpr reverse_iterator  operator++(int);
        constexpr reverse_iterator& operator--();
        constexpr reverse_iterator  operator--(int);

        constexpr reverse_iterator  operator+ (difference_type n) const;
        constexpr reverse_iterator& operator+=(difference_type n);
        constexpr reverse_iterator  operator- (difference_type n) const;
        constexpr reverse_iterator& operator-=(difference_type n);
        constexpr /* unspecified */ operator[](difference_type n) const;

        friend constexpr iter_rvalue_reference_t<Iter>
          iter_move(const reverse_iterator& i) noexcept(/* see description */);
        template<indirectly_swappable<Iter> Iter2>
          friend constexpr void
            iter_swap(const reverse_iterator& x,
                      const reverse_iterator<Iter2>& y) noexcept(/* see description */);

      protected:
        Iter current;
      };
    }

#### Class template std::back_insert_iterator

    namespace std {
      template<class Container>
      class back_insert_iterator {
      protected:
        Container* container = nullptr;

      public:
        using iterator_category = output_iterator_tag;
        using value_type        = void;
        using difference_type   = ptrdiff_t;
        using pointer           = void;
        using reference         = void;
        using container_type    = Container;

        constexpr back_insert_iterator() noexcept = default;
        constexpr explicit back_insert_iterator(Container& x);
        constexpr back_insert_iterator& operator=(const typename Container::value_type& value);
        constexpr back_insert_iterator& operator=(typename Container::value_type&& value);

        constexpr back_insert_iterator& operator*();
        constexpr back_insert_iterator& operator++();
        constexpr back_insert_iterator  operator++(int);
      };
    }

#### Class template std::front_insert_iterator

    namespace std {
      template<class Container>
      class front_insert_iterator {
      protected:
        Container* container = nullptr;

      public:
        using iterator_category = output_iterator_tag;
        using value_type        = void;
        using difference_type   = ptrdiff_t;
        using pointer           = void;
        using reference         = void;
        using container_type    = Container;

        constexpr front_insert_iterator(Container& x) noexcept = default;
        constexpr explicit front_insert_iterator(Container& x);
        constexpr front_insert_iterator&
          operator=(const typename Container::value_type& value);
        constexpr front_insert_iterator& operator=(typename Container::value_type&& value);

        constexpr front_insert_iterator& operator*();
        constexpr front_insert_iterator& operator++();
        constexpr front_insert_iterator  operator++(int);
      };
    }

#### Class template std::insert_iterator

    namespace std {
      template<class Container>
      class insert_iterator {
      protected:
        Container* container = nullptr;
        ranges::iterator_t<Container> iter = ranges::iterator_t<Container>();

      public:
        using iterator_category = output_iterator_tag;
        using value_type        = void;
        using difference_type   = ptrdiff_t;
        using pointer           = void;
        using reference         = void;
        using container_type    = Container;

        insert_iterator() = default;
        constexpr insert_iterator(Container& x, ranges::iterator_t<Container> i);
        constexpr insert_iterator& operator=(const typename Container::value_type& value);
        constexpr insert_iterator& operator=(typename Container::value_type&& value);

        constexpr insert_iterator& operator*();
        constexpr insert_iterator& operator++();
        constexpr insert_iterator& operator++(int);
      };
    }

#### Class template std::move_iterator

    namespace std {
      template<class Iter>
      class move_iterator {
      public:
        using iterator_type     = Iter;
        using iterator_concept  = /* see description */;
        using iterator_category = /* see description */;
        using value_type        = iter_value_t<Iter>;
        using difference_type   = iter_difference_t<Iter>;
        using pointer           = Iter;
        using reference         = iter_rvalue_reference_t<Iter>;

        constexpr move_iterator();
        constexpr explicit move_iterator(Iter i);
        template<class U> constexpr move_iterator(const move_iterator<U>& u);
        template<class U> constexpr move_iterator& operator=(const move_iterator<U>& u);

        constexpr iterator_type base() const &;
        constexpr iterator_type base() &&;
        constexpr reference operator*() const;
        constexpr pointer operator->() const;

        constexpr move_iterator& operator++();
        constexpr auto operator++(int);
        constexpr move_iterator& operator--();
        constexpr move_iterator operator--(int);

        constexpr move_iterator operator+(difference_type n) const;
        constexpr move_iterator& operator+=(difference_type n);
        constexpr move_iterator operator-(difference_type n) const;
        constexpr move_iterator& operator-=(difference_type n);
        constexpr reference operator[](difference_type n) const;

        template<sentinel_for<Iter> S>
          friend constexpr bool
            operator==(const move_iterator& x, const move_sentinel<S>& y);
        template<sized_sentinel_for<Iter> S>
          friend constexpr iter_difference_t<Iter>
            operator-(const move_sentinel<S>& x, const move_iterator& y);
        template<sized_sentinel_for<Iter> S>
          friend constexpr iter_difference_t<Iter>
            operator-(const move_iterator& x, const move_sentinel<S>& y);
        friend constexpr iter_rvalue_reference_t<Iter>
          iter_move(const move_iterator& i)
            noexcept(noexcept(ranges::iter_move(i.current)));
        template<indirectly_swappable<Iter> Iter2>
          friend constexpr void
            iter_swap(const move_iterator& x, const move_iterator<Iter2>& y)
              noexcept(noexcept(ranges::iter_swap(x.current, y.current)));

      private:
        Iter current;     // exposition only
      };
    }

#### Class template std::move_sentinel

    namespace std {
      template<semiregular S>
      class move_sentinel {
      public:
        constexpr move_sentinel();
        constexpr explicit move_sentinel(S s);
        template<class S2>
          requires convertible_to<const S2&, S>
            constexpr move_sentinel(const move_sentinel<S2>& s);
        template<class S2>
          requires assignable_from<S&, const S2&>
            constexpr move_sentinel& operator=(const move_sentinel<S2>& s);

        constexpr S base() const;
      private:
        S last;     // exposition only
      };
    }

#### Class template std::common_iterator

    namespace std {
      template<input_or_output_iterator I, sentinel_for<I> S>
        requires (!same_as<I, S> && copyable<I>)
      class common_iterator {
      public:
        constexpr common_iterator() = default;
        constexpr common_iterator(I i);
        constexpr common_iterator(S s);
        template<class I2, class S2>
          requires convertible_to<const I2&, I> && convertible_to<const S2&, S>
            constexpr common_iterator(const common_iterator<I2, S2>& x);

        template<class I2, class S2>
          requires convertible_to<const I2&, I> && convertible_to<const S2&, S> &&
                   assignable_from<I&, const I2&> && assignable_from<S&, const S2&>
            common_iterator& operator=(const common_iterator<I2, S2>& x);

        decltype(auto) operator*();
        decltype(auto) operator*() const
          requires dereferenceable<const I>;
        decltype(auto) operator->() const
          requires /* see description */;

        common_iterator& operator++();
        decltype(auto) operator++(int);

        template<class I2, sentinel_for<I> S2>
          requires sentinel_for<S, I2>
        friend bool operator==(
          const common_iterator& x, const common_iterator<I2, S2>& y);
        template<class I2, sentinel_for<I> S2>
          requires sentinel_for<S, I2> && equality_comparable_with<I, I2>
        friend bool operator==(
          const common_iterator& x, const common_iterator<I2, S2>& y);

        template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2>
          requires sized_sentinel_for<S, I2>
        friend iter_difference_t<I2> operator-(
          const common_iterator& x, const common_iterator<I2, S2>& y);

        friend constexpr decltype(auto) iter_move(const common_iterator& i)
          noexcept(noexcept(ranges::iter_move(declval<const I&>())))
            requires input_iterator<I>;
        template<indirectly_swappable<I> I2, class S2>
          friend void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y)
            noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));

      private:
        variant<I, S> v_;   // exposition only
      };

      template<class I, class S>
      struct incrementable_traits<common_iterator<I, S>> {
        using difference_type = iter_difference_t<I>;
      };

      template<input_iterator I, class S>
      struct iterator_traits<common_iterator<I, S>> {
        using iterator_concept = /* see description */;
        using iterator_category = /* see description */;
        using value_type = iter_value_t<I>;
        using difference_type = iter_difference_t<I>;
        using pointer = /* see description */;
        using reference = iter_reference_t<I>;
      };
    }

#### Class std::default_sentinel_t

    namespace std {
      struct default_sentinel_t { };
    }

#### Class template std::counted_iterator

    namespace std {
      template<input_or_output_iterator I>
      class counted_iterator {
      public:
        using iterator_type = I;

        constexpr counted_iterator() = default;
        constexpr counted_iterator(I x, iter_difference_t<I> n);
        template<class I2>
          requires convertible_to<const I2&, I>
            constexpr counted_iterator(const counted_iterator<I2>& x);

        template<class I2>
          requires assignable_from<I&, const I2&>
            constexpr counted_iterator& operator=(const counted_iterator<I2>& x);

        constexpr I base() const & requires copy_constructible<I>;
        constexpr I base() &&;
        constexpr iter_difference_t<I> count() const noexcept;
        constexpr decltype(auto) operator*();
        constexpr decltype(auto) operator*() const
          requires dereferenceable<const I>;
        constexpr auto operator->() const noexcept
          requires contiguous_iterator<I>;

        constexpr counted_iterator& operator++();
        decltype(auto) operator++(int);
        constexpr counted_iterator operator++(int)
          requires forward_iterator<I>;
        constexpr counted_iterator& operator--()
          requires bidirectional_iterator<I>;
        constexpr counted_iterator operator--(int)
          requires bidirectional_iterator<I>;

        constexpr counted_iterator operator+(iter_difference_t<I> n) const
          requires random_access_iterator<I>;
        friend constexpr counted_iterator operator+(
          iter_difference_t<I> n, const counted_iterator& x)
            requires random_access_iterator<I>;
        constexpr counted_iterator& operator+=(iter_difference_t<I> n)
          requires random_access_iterator<I>;

        constexpr counted_iterator operator-(iter_difference_t<I> n) const
          requires random_access_iterator<I>;
        template<common_with<I> I2>
          friend constexpr iter_difference_t<I2> operator-(
            const counted_iterator& x, const counted_iterator<I2>& y);
        friend constexpr iter_difference_t<I> operator-(
          const counted_iterator& x, default_sentinel_t);
        friend constexpr iter_difference_t<I> operator-(
          default_sentinel_t, const counted_iterator& y);
        constexpr counted_iterator& operator-=(iter_difference_t<I> n)
          requires random_access_iterator<I>;

        constexpr decltype(auto) operator[](iter_difference_t<I> n) const
          requires random_access_iterator<I>;

        template<common_with<I> I2>
          friend constexpr bool operator==(
            const counted_iterator& x, const counted_iterator<I2>& y);
        friend constexpr bool operator==(
          const counted_iterator& x, default_sentinel_t);

        template<common_with<I> I2>
          friend constexpr strong_ordering operator<=>(
            const counted_iterator& x, const counted_iterator<I2>& y);

        friend constexpr decltype(auto) iter_move(const counted_iterator& i)
          noexcept(noexcept(ranges::iter_move(i.current)))
            requires input_iterator<I>;
        template<indirectly_swappable<I> I2>
          friend constexpr void iter_swap(const counted_iterator& x,
                                          const counted_iterator<I2>& y)
            noexcept(noexcept(ranges::iter_swap(x.current, y.current)));

      private:
        I current = I();                    // exposition only
        iter_difference_t<I> length = 0;    // exposition only
      };

      template<input_iterator I>
      struct iterator_traits<counted_iterator<I>> : iterator_traits<I> {
        using pointer = void;
      };
    }

#### Class std::unreachable_sentinel_t

    namespace std {
      struct unreachable_sentinel_t {
        template<weakly_incrementable I>
          friend constexpr bool operator==(unreachable_sentinel_t, const I&) noexcept
          { return false; }
      };
    }

#### Class template std::istream_iterator

    namespace std {
      template<class T, class CharT = char, class Traits = char_traits<CharT>,
               class Distance = ptrdiff_t>
      class istream_iterator {
      public:
        using iterator_category = input_iterator_tag;
        using value_type        = T;
        using difference_type   = Distance;
        using pointer           = const T*;
        using reference         = const T&;
        using char_type         = CharT;
        using traits_type       = Traits;
        using istream_type      = basic_istream<CharT, Traits>;

        constexpr istream_iterator();
        constexpr istream_iterator(default_sentinel_t);
        istream_iterator(istream_type& s);
        istream_iterator(const istream_iterator& x) = default;
        ~istream_iterator() = default;
        istream_iterator& operator=(const istream_iterator&) = default;

        const T& operator*() const;
        const T* operator->() const;
        istream_iterator& operator++();
        istream_iterator  operator++(int);

        friend bool operator==(const istream_iterator& i, default_sentinel_t);

      private:
        basic_istream<CharT, Traits>* in_stream; // exposition only
        T value;                                 // exposition only
      };
    }

#### Class template std::ostream_iterator

    namespace std {
      template<class T, class CharT = char, classTraits = char_traits<CharT>>
      class ostream_iterator {
      public:
        using iterator_category = output_iterator_tag;
        using value_type        = void;
        using difference_type   = ptrdiff_t;
        using pointer           = void;
        using reference         = void;
        using char_type         = CharT;
        using traits_type       = Traits;
        using ostream_type      = basic_ostream<CharT, Traits>;

        constexpr ostreambuf_iterator() noexcept = default;
        ostream_iterator(ostream_type& s);
        ostream_iterator(ostream_type& s, const CharT* delimiter);
        ostream_iterator(const ostream_iterator& x);
        ~ostream_iterator();
        ostream_iterator& operator=(const ostream_iterator&) = default;
        ostream_iterator& operator=(const T& value);

        ostream_iterator& operator*();
        ostream_iterator& operator++();
        ostream_iterator& operator++(int);

      private:
        basic_ostream<CharT, Traits>* out_stream = nullptr;          // exposition only
        const CharT* delim = nullptr;                                // exposition only
      };
    }

#### Class template std::istreambuf_iterator

    namespace std {
      template<class CharT, class Traits = char_traits<CharT>>
      class istreambuf_iterator {
      public:
        using iterator_category = input_iterator_tag;
        using value_type        = CharT;
        using difference_type   = typename Traits::off_type;
        using pointer           = /* unspecified */;
        using reference         = CharT;
        using char_type         = CharT;
        using traits_type       = Traits;
        using int_type          = typename Traits::int_type;
        using streambuf_type    = basic_streambuf<CharT, Traits>;
        using istream_type      = basic_istream<CharT, Traits>;

        class proxy;                          // exposition only

        constexpr istreambuf_iterator() noexcept;
        constexpr istreambuf_iterator(default_sentinel_t) noexcept;
        istreambuf_iterator(const istreambuf_iterator&) noexcept = default;
        ~istreambuf_iterator() = default;
        istreambuf_iterator(istream_type& s) noexcept;
        istreambuf_iterator(streambuf_type* s) noexcept;
        istreambuf_iterator(const proxy& p) noexcept;
        istreambuf_iterator& operator=(const istreambuf_iterator&) noexcept = default;
        CharT operator*() const;
        istreambuf_iterator& operator++();
        proxy operator++(int);
        bool equal(const istreambuf_iterator& b) const;

        friend bool operator==(const istreambuf_iterator& i, default_sentinel_t s);

      private:
        streambuf_type* sbuf_;                // exposition only
      };

      template<class CharT, class Traits>
      class istreambuf_iterator<CharT, Traits>::proxy { // exposition only
        CharT keep_;
        basic_streambuf<CharT, Traits>* sbuf_;
        proxy(CharT c, basic_streambuf<CharT, Traits>* sbuf)
          : keep_(c), sbuf_(sbuf) { }
      public:
        CharT operator*() { return keep_; }
      };
    }

#### Class template std::ostreambuf_iterator

    namespace std {
      template<class CharT, class Traits = char_traits<CharT>>
      class ostreambuf_iterator {
      public:
        using iterator_category = output_iterator_tag;
        using value_type        = void;
        using difference_type   = ptrdiff_t;
        using pointer           = void;
        using reference         = void;
        using char_type         = CharT;
        using traits_type       = Traits;
        using streambuf_type    = basic_streambuf<CharT, Traits>;
        using ostream_type      = basic_ostream<CharT, Traits>;

        constexpr ostreambuf_iterator() noexcept = default;
        ostreambuf_iterator(ostream_type& s) noexcept;
        ostreambuf_iterator(streambuf_type* s) noexcept;
        ostreambuf_iterator& operator=(CharT c);

        ostreambuf_iterator& operator*();
        ostreambuf_iterator& operator++();
        ostreambuf_iterator& operator++(int);
        bool failed() const noexcept;

      private:
        streambuf_type* sbuf_ = nullptr;    // exposition only
      };
    }

#### Class template std::iterator

    namespace std {
      template<class Category, class T, class Distance = ptrdiff_t,
               class Pointer = T*, class Reference = T&>
      struct iterator {
        typedef Category  iterator_category;
        typedef T         value_type;
        typedef Distance  difference_type;
        typedef Pointer   pointer;
        typedef Reference reference;
      };
    }

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 349 | C++98 | the exposition-only member `delim` of std::ostream_iterator had type const char* | corrected to const CharT*

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